1. Field of the Invention
The present invention relates to an apparatus for controlling an exhaust system of an internal-combustion engine which is adapted to control the natural frequency of the exhaust system in accordance with the operating conditions of the engine in order to raise the intake efficiency thereof over a wide operation range, from low speed to high speed, thereby allowing a high output to be offered at all times.
2. Description of the Prior Art
Hitherto, a typical internal-combustion engine contrives to increase the output by such a method that the air-fuel mixture emerging into the exhaust pipe is pushed back into the cylinder by applying to the exhaust port portion a positive pressure of pulsating waves of the combustion gas reflected from the opening of the exhaust pipe in the end of the exhaust stroke in the high-speed operation. However, the speeds of the engine at which the output can be increased are limited to the high-speed operation region in the case where the shape and dimensions of the exhaust pipe are set for the high-speed operation region, so that when the engine is operating at low speed a negative pressure of the pulsating waves acts on the exhaust port portion to suck the air-fuel mixture, resulting in a reduction of output.
Therefore, such a method is considered to improve the output performance of the engine as follows. Namely, a resonance chamber with its terminating end closed is communicated with an intermediate portion of the exhaust passage of an internal-combustion engine and is provided at its inlet with a stop valve the opening/closing operation of which is controlled by a controller. By making the resonance chamber operative through the opening operation of the stop valve, the natural frequency of the exhaust passage is lowered as if the exhaust passage were elongated, thereby allowing the pulsating effect in the exhaust passage to fit to the low- and intermediate-speed operation regions of the engine. On the other hand, by making the resonance chamber inoperative through the closing operation of the stop valve, the natural frequency of the exhaust passage is raised as if the exhaust passage were reduced in length, thereby allowing the pulsating effect in the exhaust passage to fit with the high-speed operation region of the engine. Thus, the pulsating effect of the exhaust passage is efficiently utilized over all the operation regions to raise the charging efficiency of the engine, thereby contriving improvement in its output performance.
In general, however, it is not easy to control the stop valve so as to effect its opening/closing operation in accordance with a predetermined operation region of the engine, and the apparatus for effecting such control usually becomes complicated and high in cost.